Foetal Development Flashcards
What happens in week one of foetal development?
Classified as week 3 of clinical pregnancy
- Determined by the time since the last menstruation
- Throughout this first week the fertilized ovum will have developed through the morula and blastocyst stages to implant within the uterine wall
what happens in week two, three and four of foetal development?
Formation of the bilaminar germ disc, consisting of epiblast and hypoblast.
- This will form the trilaminar disc through gastrulation in week three
- By the end of the fourth week, neurulation will have occurred, and the flat trilaminar disc will have undergone cephalo-caudal and lateral folding to form the cylindrical embryo
two phases of week three development
- The embryonic period is from week 3-8 post fertilization (week 5-10 in terms of clinical pregnancy)
- This period is where organogenesis occurs, establishing the main organ systems - The foetal period is from week 9-38 post fertilisation (week 11-40 in terms of clinical pregnancy)
- This period is where tissues and organs mature and grow
Three types of birth defects
Birth defects are defined as developmental disorders present at birth
- Structural defects are congenital anomalies
- Functional defects are organ dysfunctions
- Metabolic defects are defects in enzymes or cells
3 forms of congenital anomalies
- Malformation occurs during embryonic development itself
- Results in incomplete or abnormal formation of structure, complete or partial absence of a structure, or an alteration of the normal configuration of a structure - Disruption is a morphological alteration of a structure that has already formed
- This is a destructive process e.g. the formation of amniotic bands wrapping around a pre-formed limb and effectively amputating it - Deformation is the result of mechanical factors external to the foetus
- e.g. talipes (bent feet and ankles) occurring due to positional restriction in the uterus
Causes of birth defects
- Majority of explainable birth defects result from multi-factorial inheritance
- Chromosomal or genetic causes of birth defects usually result in multi-organ involvement and will generally cause significant or lethal defects
- Environmental - exposure to teratogens
What is a teratogen?
A teratogen is defined as an agent that can cause or predispose to a birth defect; this will usually result in the involvement of a single system or only a few systems
If teratogenic insult occurs before the onset of the embryonic period (<2 weeks) it will either have no effect or cause miscarriage
If it occurs in the foetal period (>9 weeks) effects will be minimal, and not usually lead to birth defect but rather have an effect on growth and functional maturation.
The main period in which teratogens will lead to birth defect is in the embryonic period, as this is the period of greatest sensitivity to malformation (different organs will have different periods of peak sensitivity)
Types of teratogen
Drugs- alcohol, cocaine, thalidomide, anti-psychotics, ACE inhibitors
Exposure to chemicals- mercury, lead, ionising radiation
Infectious agents- rubella
Maternal factors- diabetes mellitus
Mechanical- malformed uterus
What is Spina Bifida
Caused by a defect in the process of Neurulation
- In primary neurulation, there is the closure of the neural plate into the neural tube
- If this fails at the caudal end it will result in spina bifida but if this occurs at the cranial end it will cause anencephaly
- In secondary neurulation, there is the formation of the tail end of the neural tube; this will also cause spina bifida.
3 classifications of spina bifida
- Occulta results from a defect in the bone surrounding the spinal cord, this will remain hidden and only be discovered incidentally, not causing any functional abnormality
- Meningocele results from a hernation of the meninges and CSF out of the spinal column defect
- Myelomeningocele results from a herniation of the meninges and spinal cord out of the defect
What is a cleft lip and palate
can be unilateral or bilateral, and may be hidden due to an effect on the palate alone.
It results from a failure of the fusion of the maxillary process and medial nasal clefts to form the upper lip, and the fusion of the palatal shelves to form the palate.
What is omphalocele?
Herniation of the bowel contents into the base of the umbilical cord; this will appear as a transparent sac of amnion attached to the umbilical ring containing herniated viscera
This occurs due to a persistence of the embryonic midgut herniation that occurs normally in foetal development
The presence of omphalocele is an indication of underlying chromosomal or other abnormalities, and should therefore be investigated futher
What is gastrochisis?
Evisceration of foetal intestine through a defect in the paraumbilical wall, generally to the right of the umbilicus
This could possibly be due to an involution of the right umbilical vein or right vitelline artery, or abnormal folding of the body wall
It is often associated with young maternal age, smoking, or drug use
Gastrochisis is less severe than omphalocele as it is not an indicator of underlying abnormality, and can often be corrected in a single surgery
Invasive tests for birth defects
A, chorionic villus sampling, foetal blood samples, and fetoscopy
These processes carry with them high risks of miscarriage and should only be performed under certain circumstances
Non-invasive tests for birth defects
Imaging such as ultrasound
